Method and composition for electroplating tin

ABSTRACT

AN AQUEOUS BATH FOR ELECTROPLATING TIN UPON METAL SURFACES CONTAINS STANNOUS ION, SULFATE RADICAL, A SURFACEACTIVE SULFATED POLYOXYALKYL CARBINAMINE AND AN IMIDAZOLINE DERIVATIVE. THE BATH HAS A HYDROGEN ION CONCENTRA TION OF ABOUT 0.9 TO 4.1 GRAMS PER LITER, AND IT OPERATES EFFECTIVELY EVER A WIDE RANGE OF CURRENT DENSITIES TO PRODUCE DESIRABLE DEPOSITS AT RELATIVELY HIGH PLATING RATES.

United States Patent U.s. c1.204-s4 21 Claims ABSTRACT OF THE DISCLOSUREAn aqueous bath. for electroplating tin upon metal surfaces containsstannous ion, sulfate radical, a surfaceactive sulfated polyoxyalkylcarbinamine and an imidazoline derivative. The bath has a hydrogen ionconcentration of about 0.9 to 4.1 grams per liter, and it operateseffectively over a wide range of current densities to produce desirabledeposits at relatively high plating rates.

BACKGROUND OF THE INVENTION A variety of baths have been employed forelectrodepositing tin upon metallic substrates, possibly the most widelyused of which are the stannous fluoborate and caustic stannatesolutions. Although these prior art baths have proven generallysatisfactory and have been widely utilized, they are usually deficientin one or more desirable operating characteristics, and the depositswhich they produce frequently fail to provide the levels of brightness,smoothness, ductility, adherance, solde-rability, porosity, or stabilityagainst ageing which are desired. Some of the baths are relativelyexpensive and many tend to be difiicult to maintain since fairlycritical control of the concentrations of the components thereof isnormally required. In addition, they often exhibit a tendency to berelatively slow and to operate properly only in fairly narrow ranges ofcurrent density.

Accordingly, it is an object of the present invention to provide a bathfor producing smooth, adherent and relatively nonporous deposits of tinwhich may also be bright or semibright.

It is also an object to provide such a bath which is relativelyinexpensive and convenient to maintain and which does not require a highlevel of control of the concentration of the components thereof withincritical limits.

Another object is to provide a bath for electrodepositing tin atrelatively high rates which is operable over a broad range of currentdensities.

A specific object is to provide a convenient and rapid method forelectroplating tin upon metallic workpieces utilizing an improvedsulfate plating bath to obtain highly desirable tin deposits atrelatively low cost.

SUMMARY OF THE INVENTION It has now been found that the foregoing andrelated objects can be readily attained in an aqueous bath containingabout 15.0 to 100.0 grams per liter of stannous ion, about 30.0 to 280.0grams per liter of sulfate ion, about 0.5 to 6.0 grams per liter of asurface-active sulfated polyoxyalkyl carbinamine, and about 0.5 to 3.0grams per liter of an imidazoline derivative. The hydrogen ionconcentration of the bath should be maintained at about 0.9 to 4.1 gramsper liter and the carbinamine and imidazoline are present in a ratio oftheir respective weights of about 0.753.0: 1.

The imidazoline derivative included in the bath corresponds to theformula:

wherein:

(1) R is an alkyl radical having 5 to 24 carbon atoms;

(2) G is a radical selected from the group consisting of OH, acid saltradicals, anionic surface active sulfate salt radicals and anionicsurface active sulfonate salt radicals;

(3) Z is a radical selected from the group consisting of COOM, CH COOM,and

(4) M is a substituent selected from the group consisting of hydrogen,alkali metals and organic bases;

(5) Y is selected from the group consisting of OR' and (6) each Rsubstituent is independently selected from the group consisting ofhydrogen, alkali metals, and (CH COOM;

(7) A is an anionic monovalent radical;

(8) n is an integer from 1 to 4; and

(9) both the groups G and CH Z are present or absent.

In the method of electroplating tin, the aqueous bath is prepared andmaintained at a temperature of about to Fahrenheit. A workpiece having ametallic surface and a suitable anode are immersed in the bath and avoltage providing a current density of about 150 to 2000 amperes persquare foot at the surface of the workpiece is applied across the anodeand workpiece to deposit tin upon the latter.

DESCRIPTION OF THE PREFERRED EMBODIMENTS The baths of the presentinvention require the stannous ion, the sulfate radical, asurface-active sulfated polyoxyalkyl carbinamine, and an imidazolinederivative for proper operation. The stannous ion and the sulfateradical may be furnished to the bath by any combination of compoundssuitable for use in tin plating solutions, but to avoid the presence ofunnecessary and possibly interfering ions, these components aredesirably introduced in the form of stannous sulfate and sulphuric acid.As is apparent, the former compound will provide both the stannous ionand the sulfate radical, whereas the latter compound provides additionalsulfate radical which is present in the bath. The use of sulphuric acidis also desirable from the standpoint of control of the hydrogen ionconcentration, although other acids providing non-interfering radicalsmay also be used for that purpose. Tin compounds other than stannoussulfate, e.g., stannous fiuoborate, may be employed, but preferably theyare used in conjunction with at least an equal weight of the sulfatecompound.

Although the amount of stannous ion present in the baths may varybetween about 15.0 and 100.0 grams per liter, it is preferably about35.0 to 80.0 grams per liter. Similarly, although the concentration ofsulfate ion may normally be between about 30.0 and 280.0, it ispreferably about 50.0 to 150.0 grams per liter. Although a broad rangeof about 0.9 to 4.1 grams per liter is suitable for the hydrogen ionconcentration, it is most desirably maintained at about 1.0 and 3.0grams per liter. If the concentration of stannous ion in the bath is toolow, the efiiciency of plating and the quality of the deposit producedwill be unsatisfactory. On the other hand, although high levels of thision permit desirable deposits to be obtained at higher currentdensities, too high a stannous ion concentration tends to result in theformation of an undesirable precipitate in the presence of relativelysmall amounts of sulphuric acid. Maintaining the sulfate radical and thehydrogen ion within the ranges of concentrations specified will ensurethat the bath operates efiiciently to produce the type of depositsdesired.

The surface-active carbinamine component In addition to the stannous ionand sulfate radical, the acid baths of the present invention require aneffective amount of a combination of surface-active agents, onenecessary member of which is an amphoteric sulfated polyoxyalkylcarbinamine which is stable under the conditions of plating. Generally,such carbinamines are produced by sulfating the terminal hydroxyl groupon the polyoxyalkyl chain, and they may be in the form of an alkalimetal salt thereof. Although the efiicacy of a particular member of thisfamily should be tested to ensure its compatibility and effectivenesswith the other components of the baths, various carbinamines having thefollowing formula have been suitable:

in which R R and R are alkyl groups having a total of 7 to 23, andpreferably 11 to 14, carbon atoms; m has a value of from 8 to 25, andpreferably from 12.5 to 17.5; and X is a substituent selected from theclass consisting of monovalent cations of hydrogen and of alkali metals.Surface active agents of this type are described in detail in U.S. Pat.No. 3,079,416, granted to Rohm & Haas Company on Feb. 26, 1963 as theassignee of John Dupre et al., and a particularly beneficial material isthe sulfated polyoxyalkyl tertcarbinamine sold by Rohm & Haas under thetrade name Triton QS-15.

Although it has been found that about 0.5 to 6.0 grams per liter of thecarbinamine component may be used, the baths preferably contain about0.75 to 3.0 grams per liter. Not only is there insufficient benefit tojustify the cost of using more than about 6 grams per liter of thismaterial, but also higher amounts tend to produce dullness and roughnessin the deposit. Use of less than about 0.5 gram per liter is generallyfound inadequate to produce the results desired. It will be appreciatedthat a combination of such carbinamines may be employed in a singlebath, or combinations of the type disclosed in Boettner Pat. No.

3,079,348, assigned to Rohm & Haas Company, may be r utilizedbenefically so long as the amount of the sulfated carbinamine is withinthe limits defined hereinbefore. Particularly good results have beenobtained utilizing a combination of the sulfated polyoxyalkylcarbinamine (such as the Rohm & Haas QS15) and about 0.06 to 0.5 gramper liter of an amine polyglycol condensate (such as that sold by Rohm &Haas Company under the designation Triton CF-32). The indicatedcombination serves not only to provide a low foaming bath, but the bathis surprisingly effective in producing a tin deposit which is ofincreased brightness as compared to baths from which the aminepolyglycol condensate is omitted.

The imidazoline derivative The second essential component of thecombination of surface-active agents is an imidazoline derivativecorresponding to the formula:

In the foregoing formula, R represents an alkyl radical having to 24carbon atoms; G is OH ion, an acid salt radical, an anionicsurface-active sulfate salt radical such 4 as preferably -OSO OR, or ananionic surface-active sulfonate salt radical; and Z is a --COOM, CHCOOM,

radical. The substituent designated M in the foregoing formulae is ahydrogen atom, an alkali metal or an organic base, and that designated Yis either an -OR or N(R') A group. Each R substituent is independentlyselected from the group consisting of hydrogen, alkali metals, and (CHCOOM; A represents an anionic monovalent radical and n represents aninteger from 1 to 4. Use of the dotted line representation for the bondsconnecting the substituents G and CH Z to the nitrogen atom indicatesthat the substituents are optionally present or absent, but it should beunderstood that'they are either both present or both absent. Desirablecompounds are provided when, in accordance with the foregoing formula, Grepresents the radical OSO OR, particularly in which R is a C to C alkylgroup, and Z is a COOM radical in which M is preferably an alkali metalcation.

Exemplary of the compounds corresponding to the foregoing formula whichare satisfactory for use in the baths described herein are2-alkyl-l-(ethyl-beta-oxypropanoic acid) imidazolines wherein the alkylgroup is capryl, undecyl or a mixture of C -C chains, and the disodiumsalt of lauroyl-cycloimdinium-lethoxyethanoic acid-Z-ethanoic acid.

Although the amount of the imidazoline constituent may be about 0.5 to3.0 grams per liter of solution, it is preferably about 0.75 to 2.0grams per liter. Less than about 0.5 gram per liter of this material isfound to be relatively ineffective, whereas more than about 3.0 grams iseconomically undesirable and tends to produce dull, rough deposits.Regardless of the specific amounts of the carbinamine and imidazolinederivatives which are utilized, the ratio of their respective weightsshould be about 0.75 to 3.0:1, and most desirably the ratio therebetweenwill be about 1.0 to 2.0:-l.0.

Other components In addition to the ions and compounds described hereinbefore, other additives may be included to enhance the operation ofthe present baths, such as auxiliary surfaceactive agents andbrighteners. However, one of the notable advantages provided by thepresent invention resides in the fact that very desirable deposits canbe produced with no additive to the bath other than those necessary tofurnish the stannous ion, the sulfate radical, the carbinamine, and theimidazoline derivative.

Exemplary of the brighteners which may be added are the aryl aldehydes,polyvinyl alcohol, gelatin, animal glue, evaporated milk, etc., andgenerally they will be added in a concentration of about 0.04 to 1.5grams per liter. Brightness may also be enhanced by the inclusion ofcertain antioxidants, which also serve to reduce the tendency forinsoluble precipitates to form in the bath. Such antioxidants includepyrocatechol, resorcinol and naphthol and may be employed in amounts ofabout 0.5 to 5.0, and preferably about 1.0 to 2.0 grams per liter. Theseadditives are particularly effective in baths used at the higher platingrates in which the stannous ion concentration is relatively high.

In addition to the foregoing, buffers, chelating agents and otheradditives commonly employed in baths of this type may also be includedif desired. Chelating agents such as citric acid, malic acid, and theaminopolyacetic acids (i.e., ethylenediamine tetraacetic acid,diethylenetriamine pentaacetic acid, nitriloacetic acid) areparticularly beneficial to improve operation of the baths and to enhancethe levels of brightness which may be obtained in the lower ranges ofcurrent density; they will normally be used in amounts of about 10.0 to50.0 grams per liter. It should be noted that the stannous ion, hydrogenion, and sulfate radical concentrations specified herein are based uponthe use of stannous sulfate and sulfuric acid, and that completedisassociation thereof is assumed. These factors should be borne in mindwhen determining the necessary amounts of other compounds which may beused in lieu thereof.

As has been indicated hereinbefore, the baths of the present inventionare operable over a wide range of cathode current densities, and morespecifically this range is about 150 to 2000 amperes per square foot(a.s.f.). However, the baths are preferably operated at currentdensities of about 200 to 1200 a.s.f. and, will be more fully describedhereinafter, the concentrations of the components of the the baths maybe varied considerably so as to obtain optimum results at the variouscurrent densities employed. At the lower current densities, i.e., about150 to 600 a.s.f., the rate at which the tin is deposited is about 0.2to 1.3 mils per minute; at the higher current densities, i.e., about 600and 2000 a.s.f., plating occurs at a rate of about 1.3 to 3.8 mils perminute. As a particular advantage, over the entire range of currentdensity, plating efiiciency may be as high as 98 percent, based upon thetheoretical rate of deposition.

Generally, the baths which provide the best results at the lower currentdensities contain a relatively low concentration of stannous ion ascompared to baths best operated in the higher ranges of currentdensities. Exemplary of the baths which are most suitable for use atabout 150 to 600 a.s.f. is one which contains about 50.0 grams per literof stannous ion, about 65.0 grams per liter of sulfate radical, about0.75 gram per liter of the carbinamine and about 1.0 gram per liter ofthe imidazoline derivative, the hydrogen ion concentration therein beingabout 1.0 gram per liter. As has been indicated previously, it isdesirable to include a small amount of a chelating agent, such as EDTA,in the bath described to extend its effectiveness for producingrelatively bright deposits to current densities at the low end of therange.

Exemplary of the baths which are most desirably operated at the highercurrent densities is one which contains about 75.0 grams per liter ofstannous ion, about 110.0 grams per liter of sulfate radical, about 3.0grams per liter of carbinamine, and about 1.5 grams per liter of theimidazoline derivative. This bath should have a hydrogen ionconcentration of about 2.0 grams per liter; most desirably, it alsocontains about 2.0 grams per liter of pyrocatechol and about 0.06 gramper liter of an amine polyglycol condensate (such as Rohm & Haas TritonCF- 32). It should be noted that, in the baths containing the higherconcentrations of stannous ion, care should be taken to avoid undulyhigh concentrations of sulfate radical because stannous sulfate is notsoluble at all concentrations in baths of this type and high sulfateconcentrations tend to produce undesirable precipitates therein.

The baths should be operated at temperatures between about 75 and 150Fahrenheit, and preferably about 90 to 120 Fahrenheit. Operation belowabout 75 Fahrenheit tends to be inefiicient and to produce undesirabledeposits, whereas temperature higher than about 150 Fahrenheit tend toproduce dull, rough and generally unacceptable deposits. As a generalrule, the higher the current density employed, the higher thetemperature necessary for good results; by the same token, the currentdensities which are permissible are higher at the more elevatedtemperatures than at the lower temperatures. As regards the exemplarybaths described hereinabove, the best temperature for operation of thebath of lower stannous ion concentration is about 100 Fahrenheit,whereas that for the bath of higher stannous ion concentration is about120 Fahrenheit.

Although the practice is not absolutely essential, the best results areobtained with the present baths if the cathode (i.e., the workpiece) ismoved relative to the bath at a high rate of speed. It will beappreciated that this may be accomplished by rotating the workpiece inthe bath or by moving it linearly at a high rate of speed therethrough,such as in wire plating operations; it may also be achieved by agitatingthe bath or by rapid circulation thereof. Without such movement thedeposits produced tend to be dark, spongy and poorly adherent, and,although the theory is not clearly understood, it is believed that thisis due to the formation of a film about the cathode through which thestannous ion must diffuse. It is believed that the relative motion tendsto decrease the effect of such a cathode film barrier by eliminating itor by decreasing its thickness, thus facilitating movement of the ionsto the cathode. The rate of relative movement may vary considerably, butmost desirably the workpiece is moved relative to the bath at a linearrate equivalent to at least about 200 feet per minute. Rates as high as800 feet per minute, and even higher, may be employed depending upon thecharacteristics of the bath and the workpiece, and the type of apparatuemployed. Any metallic substrate or metal-surfaced article which can beplated with tin using prior art baths may be coated in accordance withthe present invention. For example, good deposits of tin may be producedupon articles of copper, nickel, iron and steel.

The best results are obtainable with these baths if relatively pure tinanodes are employed. One important feature with regard to the anode isits surface area relative to that of the cathode, since it has beenfound that current densities at the anode in excess of about a.s.f.cause excessive anode polarization with a resultant sharp decline incurrent efficiency. The conductivity of these baths is good and they areoperative at relatively low voltages to produce good deposits. However,since these factors will vary somewhat according to the components andthe concentrations thereof in the bath, some limited experimentation maybe desirable to obtain optimum results for a given composition,installation and part, as is well known in the art.

Filtration of the bath is not essential but is normally beneficialparticularly when contamination is encountered from air-borne impuritiesand by carry-over from other operations. For this purpose, variousfilter media including fabrics, porous stoneware and other conventionalmaterials may be utilized. The need for correction of the depletion ofthe various components of the bath is best determined by a periodicquantitative analysis for the several components, the frequency thereofbeing a function of the size of the bath and the plating rates employed.The baths are relatively low in cost and utilize additives which may beused in relatively non-critical ranges of concentration, so that theymay be readily maintained in condition for proper operation. Moreover,the baths and the components are easily stored as stable solutions,which may be readily prepared.

Illustrative of the efiicacy of the present invention are the followingspecific examples wherein all percentages are on a weight basis unlessotherwise indicated.

EXAMPLE ONE An aqueous bath is prepared containing about grams per literof stannous sulfate, about 45 grams per liter of sulfuric acid, aboutone gram per liter of 2-alkyl-1-(ethylbeta-oxypropanoicacid)-imidazoline, sold by Mona Industries, Inc. under the tradenameMonateric CYA, and about 0.75 gram per liter of sulfated polyoxyalkyltertcarbinamine, sold by Rohm and Haas under the tradename Triton QS-15;it has a hydrogen ion concentration of about 0.9 gram per liter. Acathode consisting of a length of copper wire and a pure tin anode areinunersed in the bath, which is heated to a temperature of about 80Fahrenheit; the cathode is rotated in the bath at a rate equivalent toabout 200 linear feet per minute. A voltage is applied across the anodeand cathode to produce a current density of about 200 a.s.f. at thecathode and to cause tin to plate thereupon, the anode being of a sizerelative to the cathode sufficient to maintain the current densitythereat below 80 a.s.f.

Thereafter, the same bath is heated to a temperature of 120 Fahrenheitand a second wire anode is immersed along with a cathode which isrotated at a rate equivalent to about 600 linear feet per minute. Thevoltage applied is sufiicient to produce a cathode current density ofabout 400 a.s.f. and the relative size of the anode and cathode aresuflicient to maintain the anode current density below 80 a.s.f.

In both instances, the deposits produced exhibit excellent adherence tothe copper wire, and the ductility, solderability and resistance toageing thereof are found to be good. The workpiece plated at a currentdensity of 400 a.s.f. is tested in accordance with ASTM Method B-33 andfound to be nonporous.

EXAMPLE TWO A bath of the same composition as that prepared inaccordance with Example One is prepared, with the sole exception thatthe 0.75 gram per liter of the carbinamine used therein is substitutedwith an equal weight of the imidazoline derivative specified so that thetotal amount of that compound in the bath amounts to 1.75 grams perliter. Plating with this bath under the conditions specified in ExampleOne produces a deposit which is rough and streaked and which isgenerally unacceptable.

A second bath is prepared comparable to the bath of Example One, thesole difference being that the quantity of imidazoline used therein issubstituted by an equal weight of the carbinamine, so that the totalamount of carbinamine contained in the bath is 1.75 grams per liter. Useof this bath under the conditions indicated in Example One results in adeposit which is very coarse and completely unacceptable.

Accordingly, it is seen by a comparison of Examples One and Two thatutilization of either the carbinamine or the imidazoline derivativealone produces unacceptable deposits. However, when these materials areused in combination in Example One in a total amount equal to the amountof each component used independently in Example Two, deposits of tin areproduced which are highly desirable.

EXAMPLE THREE An aqueous bath is prepared as in Example One utilizing120 grams per liter of stannous sulfate, about 45 grams per liter ofsulfuric acid, about 1.0 gram per liter of the same carbinamine, andabout 0.75 gram per liter of the same imidazoline derivative. This bath,maintained at a temperature of about 120 Fahrenheit and operated undercathode current densities of 1200 and 1800 a.s.f. produces semi-brightdeposits upon a length of steel tubing one inch in diameter spun at 2250rpm. during plating. Adding about 1.0 gram per liter of pyrocatechol tothe bath increases the brightness of the deposit, and about 0.1 gram perliter of an amine polyglycol condensate (Rohm and Haas Triton (IF-32)enhances the brightness even further while also reducing the tendencyfor foaming the bath.

EXAMPLE FOUR Baths similar to those prepared in accordance with ExampleThree are prepared, substituting about 40 grams per liter of thestannous sulfate used therein with an equal amount of stannousfiuoborate and utilizing lauroyl cycloimidinium-l-ethoxy ethanoicacid-2-ethanoic acid-disodium salt (Miranol C2M sold by Miranol ChemicalCompany) instead of the imidazoline derivative specified. In allinstances the deposits obtained are of comparable quality.

EXAMPLE FIVE An aqueous bath is prepared as in Example One by dissolvingin water about 180 grams per liter of stannous sulfate, about 18 gramsper liter of sulfuric acid, about 1.0 gram per liter of the samecarbinamine, about 0.75

gram per liter of the same imidazoline derivative and about 1.0 gram perliter of pyrocatechol. The bath is heated and operated as in ExampleOne, with the exception that the voltage applied produces currentdensities of 900 to 1200 a.s.f. at the cathode; in each instance thetemperature employed is Fahrenheit. Although dull deposits are obtainedat both plating rates, they are very smooth; increasing theconcentration of pyrocatechol to 3.0 grams per liter producessemi-bright deposits under the same conditions.

Thus, it can be seen that the present invention provides a bath forproducing a smooth, adherent and relatively non-porous deposit of tinwhich may in addition be bright or semi-bright. The bath is relativelyinexpensive and is convenient to maintain since it does not require ahigh level of control of the concentration of the components thereofwithin critical limits. The bath may be used for depositing tin atrelatively high rates and over a broad range of current density so as toaiford operating and economic advantages.

Having thus described the invention, we claim:

1. An aqueous acid bath for producing an adherent electroplated tindeposit comprising about 15.0 to 100.0 grams per liter of stannous ion;about 30.0 to 280.0 grams per liter of sulfate radical; about 0.5 to6.0- grams per liter of a surface-active sulfated polyoxyalkylcarbinamine; about 0.5 to 3.0 grams per liter of a surface-activeimidazoline derivative; about 0.9 to 4.1 grams per liter of hydrogenion, said carbinamine and imidazoline derivative being present in aratio of their respective weights of about 0.75 to 3.0: 1.0.

2. The bath of claim 1 comprising about 35.0 to 80.0 grams per liter ofstannous ion, about 50.0 to 150.0 grams per liter of sulfate radical,about 0.75 to 3.0 grams per liter of said carbinamine and about 0.75 to2.0 grams per liter of said imidazoline derivative, said ratio ofcarbinamine to imidazoline being about 1.0- to 2.0210, and the hydrogenion concentration in said bath being about 1.0 to 3.0 grams per liter.

3. The bath of claim 1 wherein at least a portion of said stannous ionis provided by stannous sulfate and wherein at least a portion of saidsulfate radical is furnished by sulphuric acid.

4. The bath of claim 1 wherein said carbinamine has has the formula:

1 Rg-()NH(C2H40) SO3X wherein R R and R are alkyl groups having a totalof 7 to 23 carbon atoms, m is a number from 8 to 25 and X is asubstituent selected from the class consisting of mono valent cations ofhydrogen and of alkali metals.

5. The bath of claim 1 wherein said imidazoline derivative correspondsto the formula:

EEC-CH9, ICHZZ Nl-CHZOHEY wherein:

(l) R is an alkyl radical having 5 to 24 carbon atoms;

(2) G is a radical selected from the group consisting of OH, acid saltradicals, anionic surface active sulfate salt radicals, and anionicsurface active sulfonate salt radicals;

(3) Z is a radical selected from the group consisting of COOM, CH COOM,and

(4) M is a substituent selected from the group con sisting of hydrogen,alkali metals and organic bases;

(5) Y is selected from the group consisting of OR and N(R') A;

(6) each R substituent is independently selected from the groupconsisting of hydrogen, alkali metals, and (CH COOM;

(7) A is an anionic monovalent radical;

(8) n is an integer from 1 to 4; and

(9) both the groups G and CH Z are present or absent.

6. The bath of claim 1 wherein said imidazoline derivative is selectedfrom the group consisting of 2-alkyl-1- (ethyl-beta-oxypropanoicacid)-imidazolines wherein the alkyl group is selected from the classconsisting of capryl, undecyl and mixtures of C C chains, and thedisodium (salt) of lauroyl-cyclo-imidinium-l-ethoxy-ethanoic acid- 2-ethanoic acid.

7. The bath of claim 1 additionally containing about 0.5 to 5.0 gramsper liter of an antioxidant selected from the group consisting ofpyrocatechol, resorcinol and naphthol.

8. The bath of claim 1 additionally containing about 0.06 to 0.5 gramper liter of an amine polyglycol condensate brightening and defoamingagent.

9. The bath of claim 1 additionally containing about 10.0 to 50.0 gramsper liter of a chelating agent selected from the group consisting ofmalic acid, citric acid and aminopolyacetic acids.

10. The bath of claim wherein said imidazoline derivative has astructure wherein G is an anionic surfaceactive sulfate salt radicalhaving the formula OSO OR in which sulfate salt radical R is an alkylgroup containing 10 to 18 carbon atoms, and wherein Z is a COOM radicalin which M is an alkali metal cation.

11. In a method of electroplating tin, the steps comprising:

(A) preparing an aqueous acid bath comprising about 15.0 to 100.0 gramsper liter of stannous ion; about 30.0 to 280.0 grams per liter ofsulfate radical; about 0.5 to 6.0 grams per liter of a surface-activesulfated polyoxyalkyl carbinamine; about 0.5 to 3.0 grams per liter of asurface-active imidazoline derivative; and about 0.9 to 4.1 grams perliter of hydrogen ion, said carbinamine and imidazoline derivative beingpresent in a ratio of their respective weights of about 0.75 to 3.0:1.0;

(B) maintaining said bath at a temperature of about 75 to 150Fahrenheit;

(C) immersing a workpiece having a metallic surface and an anode in saidbath; and

(D) applying a voltage across said anode and workpiece to deposit tin onsaid metallic surface, said voltage providing a current density of about150 to 2000 amperes per square foot at the workpiece.

12. The method of claim 11 wherein relative movement is establishedbetween said bath and the surface of said workpiece.

13. The method of claim 11 wherein said bath comprises about 35.0 to80.0 grams per liter of stannous ion, about 50.0 to 150.0 grams perliter of sulfate radical, about 0.75 to 3.0 grams per liter of saidcarbinamine, about 0.75 to 2.0 grams per liter of said imidazolinederivative and about 1.0 to 3.0 grams per liter of hydrogen ion, saidratio of carbinamine to imidazoline being about 1.0 to 2021.0.

14. The method of claim 11 wherein at least a portion of said stannousion is provided by stannous sulfate and 10 wherein at least a portion ofsaid sulfate radical i furnished by sulphuric acid.

15. The method of claim 11 wherein said carbinamine has the formula:

wherein R R and R are alkyl groups having a total of 7 to 23 carbonatoms, m is a number from 8 to 25 and X is a substituent selected fromthe class consisting of monovalent cations of hydrogen and of alkalimetals.

16. The method of claim 11 wherein said imidazoline derivativecorresponds to the formula:

wherein:

(1) R is an alkyl radical having 5 to 24 carbon atoms;

(2) G is a radical selected from the group consisting of OH, acid saltradicals, anionic surface active sulfate salt radicals, and anionicsurface active sulfonate salt radicals;

(3) Z is a radical selected from the group consisting of COOM, CH COOM,and

(4) M is a substituent selected from the group consisting of hydrogen,alkali metals and organic bases;

(5) Y is selected from the group consisting of OR and N(R) A;

(6) each R substituent is independently selected from the groupconsisting of hydrogen, alkali metals, and (CH COOM;

(7 A is an anionic monovalent radical;

(8) n is an integer from 1 to 4; and

(9) both the groups G and CH Z are present or absent.

17. The method of claim 11 wherein said voltage applied provides acurrent density of about 200 to 1200 amperes per square foot and whereinsaid temperature is maintained at about to Fahrenheit.

18. The method of claim 11 wherein tin is deposited upon said workpieceat the rate of about 0.2 to 3.8 mils per minute.

19. The method of claim 11 wherein said anode has a surface arearelative to that of said workpiece such as to provide a current densityat said anode less than about 80 amperes per square foot.

20. The method of claim 12 wherein said movement is created by rapidmovement of said workpiece.

21. The method of claim 12 wherein said movement is at a rate equivalentto at least about 200 linear feet per minute.

References Cited UNITED STATES PATENTS 1/1968 Korpiun et al. 204545/1966 Hart 20454

